Gene csn encoding chitosanase of bacterial strain Bacillus cereus HN90 isolated from Vietnam was amplified by PCR using specific primers that were designed on the basis of similar sequences encoding csn of other B. cereus strains deposited on GenBank (NCB!). The experimental results revealed that csn gene sequence of strain HN90, consists of 1362 nucleotides coding for a 453-amino-acid protein, showed high homology (appromixately 96 percent-99 percent) with that from other B. cereus strains. The csn gene sequence in the present study was deposited onto GenBank under the accession number FJ682391. Gene csn was then subcloned into expression vector pET22b(+) to yield plasmid pET22b::csn. The obtained plasmid was then transferred into host bacterium to generate recombinant strain E. coli BL21 [pET22b::csn]. When expressing recombinant chitosanase in E. coli BI21(DE3), the optimal conditions were found by inducing with 0.4 mM IPTG at bacterial cell density at OD600nm of 0.6 unit for 5h at 25°C. On the SDS-P AGE gel, the molecular mass of the overexpressed chitosanase showed about 44 kDa, corresponding closely to the expected one. Under the optimized conditions, the activity of recombinant chitosanase reached up 519.0 U (mg of protein)-1 which was about 2.5-fold higher than that of wide-type strain B. cereus HN90.